Hydraulic control valves of the type normally operated by application of hydraulic pressure at opposite ends of a valve spool commonly also include a control handle or lever for use in manually controlling the position of the valve spool. The control handles are used to override pilot valves.
An example of the use of such control handles is in valves used to control movement of an articulated boom of an aerial lift. Aerial lifts commonly include three or more control valves controlling the supply of hydraulic fluid to hydraulic cylinders and hydraulic motors. These hydraulic cylinders and hydraulic motors are used to control the movement of the articulated boom. The control valves are normally mounted at the base of the boom, and a remote control device is mounted at the operator's platform. The operator uses the remote control device to control pilot valves in turn controlling operation of the control valves. The pilot valves control the delivery of hydraulic fluid to the opposite ends of the valve spools of the control valves and thereby control the relative position of the valve spools.
It is also necessary to provide means for controlling the position of the platform from the ground level, and this means must be capable of overriding the remote control lever at the platform. In the event the remote control handle at the platform jams or malfunctions, an operator at the ground level can control movement of the aerial device by manipulation of the override control handles.
One of the prior art arrangements for use in providing override control for a hydraulic control valve is illustrated in Myers U.S. Pat. No. 2,946,196, issued July 26, 1960.
Other arrangements for providing an override control is illustrated in applicant's U.S. patent application Ser. No. 762,628, filed Aug. 5, 1985 and title Manual Override Control Handle Selectively Engageable with Valve Spool.